Mass spectrometers have become common tools in chemical analysis. Generally, mass spectrometers operate by separating ionized atoms or molecules based on differences in their mass-to-charge ratio (m/e) and thereafter, detecting ions of different ratios. A variety of mass spectrometer devices are commonly in use, including ion traps, quadrupole mass filters, and magnetic sector devices.
The general steps in performing a mass-spectrometric analysis are: (1) create gas-phase ions from a sample, wherein gaseous samples may first be separated by a gas chromatograph (GC) before undergoing analysis in a mass spectrometer; (2) separate the ions in space or time based on their mass-to-charge ratio; and (3) measure the quantity of ions of each selected mass-to-charge ratio. Thus, in general, a mass spectrometer system consists of an ion source, a mass-selective analyzer, and an ion detector. In the mass-selective analyzer, magnetic and electric fields may be used, either separately or in combination, to separate the ions based on their mass-to-charge ratio. Hereinafter, the mass-selective analyzer portion of a mass spectrometer system will be referred to as a mass spectrometer.
An ion trap mass spectrometer uses electrodes to contain or "trap" the ions in a small volume, and then selectively ejects the ions from that volume to a detector. There are two primary types of ion trap mass analyzers: a three-dimensional quadrupole ion trap; and an ion cyclotron resonance (ICR) ion trap. A quadrupole ion trap contains the ions formed from a sample material in the trap and uses DC and RF electric fields to manipulate the ions to select a desired mass-to-charge ratio for detection and measurement of the number of ions. Typically, a quadrupole ion trap mass analyzer consists of a ring electrode separating two (end-cap) electrodes. The surfaces of both the ring and end-cap electrodes are generally hyperbolic in cross-section. The RF and DC potentials on the electrodes can be scanned to eject ions of a specific mass-to-charge ratio from the trap, where they are detected and counted. An ICR type ion trap uses magnetic confinement in the radial direction and DC confinement in the axial direction to contain the ions in the trap.
The sample material from which the ions are formed can be directed into the interior of the ion trap and ionized within the region between the trapping electrodes. Alternately, the sample can be introduced into an ion source external to the trapping region, ionized, and the resulting sample ions injected into the ion trap.
The ions formed within or external to the ion trap are typically produced as a result of either an electron ionization (EI) or chemical ionization (CI) process. In the EI method, a beam of electrons is directed into the gas-phase sample. Electrons collide with neutral sample molecules, producing ions of the sample molecule, or of fragments of the molecules.
One prior art ion source for producing electron ionization inside of an ion trap uses pulsed, low energy (.about.11 eV) electrons, which are injected into the interior of the ion trap electrode structure through a hole in an end-cap electrode. The RF trapping field then accelerates the electrons to a kinetic energy sufficient to fragment the neutral sample molecule(s) and form ions by electron ionization. Such a device is described by Stafford et al. in U.S. Pat. No. 4,540,884.
Bier et al. (U.S. Pat. No. 5,756,996) describes an external EI ion source that creates sample ions outside of the trap which are then injected into the trapping region. External sources such as that described in Bier et al. typically include a magnet with its field oriented along the axis of ionization to cause electrons to travel in small spiral trajectories. The resulting electron beam traverses the ionization region. Bier et al. teaches a method of controlling the energy of the electrons injected into the ion-forming volume of the external ion source. The Bier method is employed to ensure that the electron beam energy is sufficient to ionize atoms and molecules in the source during a specified ionizing period, and insufficient to ionize or excite helium (which is conventionally used as a carrier gas) at other times.
However, a disadvantage of the prior art method of internal ionization described in Stafford et al. is that the large surface area of the trap electrodes necessarily comes into contact with the sample introduced within them for ionization. The large surface area of the electrodes often reduces the sensitivity when certain types of samples are analyzed, such as highly polar compounds. This is believed to be due to the absorption of the sample on the metal electrodes. The simultaneous presence of the neutral sample molecules and the charged ion fragments within the ion trap can also cause undesired ion/molecule reactions.
In contrast, the use of an external ion source with a substantially reduced volume and electrode surface area greatly reduces the problem of sample absorption and ion/molecule reactions. An external ion source also ensures that only the ions injected into the ion trap will be present in the trap, and that the neutral sample molecules remain in the external source until they are removed by a vacuum pump. Undesired ion-molecule reactions within the ion trap can thus be substantially eliminated by using an external source.
However, a significant disadvantage of a conventional external ion source is the rate at which it becomes contaminated by sample molecules that are dissociated by collisions with electrons. In this regard, reducing the electron energy as taught by Bier et al. will reduce the photon noise caused by electron impact ionization of neutral molecules and the background of helium carrier gas used for GC. However, a chemical bond can be broken with an electron energy of only a few electron volts, which is a level far below the energy threshold for noise formation or electron impact ionization. This means that the Bier et al. approach is capable of reducing the photon noise without satisfactorily addressing the molecule dissociation problem. This is because contamination arising from sample molecule dissociation can occur without introducing significant photon noise into the measurements.
However, the method of Bier et al. cannot be used to reduce the electron energy to zero in order to reduce this potential contamination. The electron emission from a heated filament is governed by the Child-Langmuir Law for space-charge limited current flow. This law states that the maximum charged current (I) that can leave a heated filament and travel to the counter electrode, which is at a potential (V), is given by I=K V.sup.3/2. Thus, the current is a strong function of the filament bias voltage, which determines the electron energy. Applying the Bier et al. approach by reducing the electron energy to a value that will prevent electron impact ionization and molecule dissociation will thus also significantly reduce the electron emission current. This result is undesirable for the following reason.
It is known to regulate the emission current for mass spectrometry applications to ensure a stable response from the sample molecules. The regulating circuits generally have a long time constant for responding to changes in the emission current. This prevents over-heating of the filament during the initial heating of the filament, when there is little or no electron emission occurring. Thus, small changes in the filament bias voltage typically cause large changes in emission currents, resulting in a long filament emission regulator circuit response time. If the filament bias voltage is too small, then the negative space charge due to the electrons will prevent any further increase in the electrons leaving the filament, as described by the Child-Langmuir Law. In this case, the emission regulator circuit will increase the heating current through the filament until the filament melts and breaks. Therefore, it is desirable to maintain a constant electron emission current from the filament and to preserve the physical integrity of the filament.
In the method of Bier et al., during the period in which ions are not to be formed, the reduction of the filament bias voltage is accompanied by an increase in the voltage applied to the electron lens. This serves to maintain an approximately constant electron energy, until the electrons pass through the electron lens. This is important because even small changes in the electron energy will cause a large variation in the filament emission current. For a space charge limited planar diode, the Child-Langmuir Law takes the form of I=K V.sup.3/2 /X.sup.2, where X is the distance between the electrodes. Thus the emission current cannot truly be kept constant by changing the voltages on two different electrodes that are located at different distances from the filament. Since the electron emission cannot remain constant during the time required for the emission regulating circuit to respond to the change in bias voltage, the number of ions formed will not be linearly proportional to the ionizing time. This is undesirable because it complicates the process of interpreting the results of the ion measurement process.
In the CI method, ion-molecule reactions are used to produce sample ions. A reagent gas (such as methane, isobutane, or ammonia) is ionized by interaction with an electron beam. A sufficiently high reagent gas pressure can produce ion-molecule reactions between the reagent gas ions and reagent gas molecules. Some of these reaction products can then react with the sample molecules to produce sample ions.
Reagent ion formation may result from a complex set of chemical reactions. In order to maintain a stable CI reaction with a sample molecule, the reagent ions must be maintained at a constant concentration. Therefore, it is desirable that the reagent ions achieve an equilibrium level before the sample ions begin to react. The equilibrium time will be different for different chemical reagent molecules, but is generally on the order of 1-10 milliseconds. Since the reagent ion/molecule reactions that are a precursor to the formation of the sample ions may require a variety of different reaction times, a stabilization time is necessary to allow the reagent ions to achieve chemical equilibrium so that the concentration of reagent ions doesn't change during the ionization time. However, the Bier et al. method teaches that the ionization period begins by increasing the electron energy to produce ionization within the ion volume of the ion source; the CI reactions start simultaneously, and ions are introduced into the ion trap. Thus, no means is provided for eliminating any undesired effects from the non-equilibrium state of reagent ions at the beginning of the ionization period.
What is desired is an ion source for use with an ion trap mass spectrometer which overcomes the noted disadvantages of conventional ion sources.